Weinglass first caught the public eye in 1970, when he and fellow attorney William Kuntsler together defended members of the radical Chicago Eight -- Rennie Davis, David Dellinger, John Froines, Tom Hayden, Abbie Hoffman, Jerry Rubin, Bobby Seale, and Lee Weiner -- who stood accused of having incited a riot at the 1968 Democratic National Convention.

Other notable Weinglass clients include: Weather Underground member and Brinks robber Kathy Boudin; Amy Carter (daughter of former President Jimmy Carter), who in 1987 was charged with criminal trespass for occupying a university building that served as a CIA recruitment center; Jane Fonda, who was subpoenaed to testify by the House Committee on Un-American Activities after she had accused American troops of war crimes during her 1972 trip to Hanoi; domestic terrorists Bill and Emily Harris of the Symbionese Liberation Army; anti-war activist Ron Kaufman; and Anthony Russo, who, along with Daniel Ellsberg, leaked classified government documents known as the Pentagon Papers.

Weinglass also drew up a 1970 trial brief for Angela Davis, who was implicated by more than twenty witnesses for her role in providing Black Panther Party members with an arsenal of weapons that they had used to kill a California judge.

In 1982 Weinglass spent some time as the lead attorney on the legal team defending Mumia Abu Jamal, who had murdered a Philadelphia police officer in 1981. Mumia later fired Weinglass and filed suit against him for allegedly having provided inadequate representation. Weinglass penned a 1995 book on his experiences as Mumia’s attorney, titled Race for Justice: Mumia Abu-Jamal’s Fight Against the Death Penalty.

Weinglass also defended Antonio Guerrero, one of the "Cuban Five" who were arrested in 1998 and charged with espionage, conspiracy to commit murder, and attempted infiltration of U.S. military bases and Cuban exile groups in South Florida. Guerrero and his compatriots were convicted on all charges in 2001.

Weinglass has long demonstrated an affinity for Communist causes. In 1968 he traveled to Cuba as a guest of its dictator, Fidel Castro; in 1972 he went to Hanoi as guest of the North Vietnamese government; and in 1985 he and Ramsey Clark visited Nicaragua as guests of its Soviet-sponsored, Castro-aligned, Marxist Sandinista regime. On other occasions, Weinglass and Clark also traveled together to Iran and the Philippines.

Since the 9/11 terrorist attacks, Weinglass has lent his name and support to a number of anti-war groups and causes. In 2003 he was a signatory to the “Statement of Conscience” crafted by Not In Our Name, a project of C. Clark Kissinger’s Revolutionary Communist Party. This document condemned not only the Bush administration’s “stark new measures of repression,” but also its “unjust, immoral, illegitimate, [and] openly imperial policy towards the world.”

Weinglass also endorsed World Can’t Wait, another Revolutionary Communist Party project that sought to organize “people living in the United States to take responsibility to stop the whole disastrous course led by the Bush administration.”

An icon of the "legal left," Weinglass in 1974 was the first recipient of the American Civil Liberties Union's Clarence Darrow Award, which is “given annually to an individual attorney for zealous criminal defense advocacy.” In 1980 he received the Humanitarian Award of the First Unitarian Church of Los Angeles. And in 1994, California Attorneys for Criminal Justice gave him their Outstanding Achievement Award.

In Weinglass’ view, the American criminal-justice system has grown increasingly and unduly harsh in recent decades. He laments that "society has become far more punitive," that the U.S. has not banned lifetime prison sentences for juveniles younger than 18, and that the American government encourages the use of "torture" against prisoners-of-war. Adds Weinglass: "I think it's a reaction to the 60s, to what most people considered an overly permissive era, which was followed by a sharp increase in crime, which was followed by a vindictive reaction, which we are still undergoing."